127 research outputs found
Single molecule studies of ligand-DNA interactions using atomic force microscopy
This thesis describes the results of experiments into the intra and inter-molecular binding of
various ligands with dsDNA via the mechanism of intercalation, principally using the
technique of atomic force microscopy (AFM). Since the description of the first AFM in the
mid 1980’s, AFM has emerged as a sensitive and versatile analytical tool, capable both of
detecting and manipulating artefacts at picometer resolutions.
In these studies, AFM imaging, supported by circular dichroism, reveals unusual
conformational changes in DNA that occur as a result of the binding of ligands that
incorporate the acridine chromophore. These changes are distinct from those observed
following the binding of other intercalators such as doxorubicin and echinomycin. Direct
measurement of the length of linear DNA strands bound to acridine based ligands reveals a
shortening of the DNA at very low ligand concentrations. This observation suggests that the
structural changes that occur in DNA following the intercalation of the acridine
chromophore are more wide ranging than previously thought and support molecular
modeling studies that have proposed that the intercalated DNA duplex exhibits
characteristics of both B and A form DNA. Variations in the conformational changes that
occur in DNA as a result of intercalation may have implications for the application of new
intercalating ligands as chemotherapeutic agents.
In addition, single molecule force spectroscopy has been used to examine the capacity of
bisintercalators to bind to DNA in an inter-molecular fashion. By stretching individual
strands of dsDNA, acridine dimers are shown to bind to separate strands of DNA. Intermolecular
binding of this kind remains an unexplored cytotoxic mechanism that may yet find
an application in vivo. This observation is supported by a novel assay that utilises the
controlled aggregation of gold nanoparticles. These nanoparticles, functionalised with DNA,
are shown to aggregate on addition of a bisintercalator. The aggregation is fully reversible
with the addition of sodium dodecylsulphate.
These force spectroscopy experiments have also uncovered a previously unobserved, intermolecular
binding mode of the peptide antibiotics echinomycin and TANDEM. In certain
circumstances, these ligands are revealed to bind exclusively to the termini of separate DNA
strands in a sequence dependent fashion. This finding may have implications for the
employment of these ligands in the nanosciences, as a tool for joining short pieces of DNA
or improving the efficiency of the enzymatic, blunt-end ligation of DNA
The Transit Light Source Effect: False Spectral Features and Incorrect Densities for M-dwarf Transiting Planets
Transmission spectra are differential measurements that utilize stellar
illumination to probe transiting exoplanet atmospheres. Any spectral difference
between the illuminating light source and the disk-integrated stellar spectrum
due to starspots and faculae will be imprinted in the observed transmission
spectrum. However, few constraints exist for the extent of photospheric
heterogeneities in M dwarfs. Here, we model spot and faculae covering fractions
consistent with observed photometric variabilities for M dwarfs and the
associated 0.3-5.5 m stellar contamination spectra. We find that large
ranges of spot and faculae covering fractions are consistent with observations
and corrections assuming a linear relation between variability amplitude and
covering fractions generally underestimate the stellar contamination. Using
realistic estimates for spot and faculae covering fractions, we find stellar
contamination can be more than larger than transit depth changes
expected for atmospheric features in rocky exoplanets. We also find that
stellar spectral contamination can lead to systematic errors in radius and
therefore the derived density of small planets. In the case of the TRAPPIST-1
system, we show that TRAPPIST-1's rotational variability is consistent with
spot covering fractions and faculae covering
fractions . The associated stellar contamination
signals alter transit depths of the TRAPPIST-1 planets at wavelengths of
interest for planetary atmospheric species by roughly 1-15 the
strength of planetary features, significantly complicating follow-up
observations of this system. Similarly, we find stellar contamination can lead
to underestimates of bulk densities of the TRAPPIST-1 planets of , thus leading to overestimates of their volatile contents.Comment: accepted for publication in Ap
Towards robust corrections for stellar contamination in JWST exoplanet transmission spectra
Transmission spectroscopy is still the preferred characterization technique
for exoplanet atmospheres, although it presents unique challenges which
translate into characterization bottlenecks when robust mitigation strategies
are missing. Stellar contamination is one of such challenges that can overpower
the planetary signal by up to an order of magnitude, and thus not accounting
for stellar contamination can lead to significant biases in the derived
atmospheric properties. Yet, accounting for stellar contamination may not be
straightforward, as important discrepancies exist between state-of-the-art
stellar models and measured spectra and between models themselves. Here we
explore the extent to which stellar models can be used to reliably correct for
stellar contamination and yield a planet's uncontaminated transmission
spectrum. We find that (1) discrepancies between stellar models can dominate
the noise budget of JWST transmission spectra of planets around stars with
heterogeneous photospheres; (2) the true number of unique photospheric spectral
components and their properties can only be accurately retrieved when the
stellar models have a sufficient fidelity; and (3) under such optimistic
circumstances the contribution of stellar contamination to the noise budget of
a transmission spectrum is considerably below that of the photon noise for the
standard transit observation setup. Therefore, we suggest (1) increased efforts
towards development of model spectra of stars and their active regions in a
data-driven manner; and (2) the development of empirical approaches for
deriving spectra of photospheric components using the observatories with which
the atmospheric explorations are carried out.Comment: 15 pages, 8 figures, 2 table
Empirically Constraining the Spectra of a Stars Heterogeneities From Its Rotation Lightcurve
Transmission spectroscopy is currently the most powerful technique to study a
wide range of planetary atmospheres, leveraging the filtering of a stars light
by a planets atmosphere rather than its own emission. However, both a planet
and its star contribute to the information encoded in a transmission spectrum
and a particular challenge relate to disentangling their contributions. As
measurements improve, the lack of fidelity of stellar spectra models present a
bottleneck for accurate disentanglement. Considering JWST and future
high-precision spectroscopy missions, we investigate the ability to derive
empirical constraints on the emission spectra of stellar surface
heterogeneities (i.e., spots and faculae) using the same facility as used to
acquire the transmission spectra intended to characterize a given atmosphere.
Using TRAPPIST-1 as a test case, we demonstrate that it is possible to
constrain the photospheric spectrum to 0.2% and the spectra of stellar
heterogeneities to within 1-5%, which will be valuable benchmarks to inform the
new generation of theoretical stellar models. Long baseline of observations
(90% of the stellar rotation period) are necessary to ensure the photon-limited
(i.e., instrument-limited) exploration of exoplanetary atmospheres via
transmission spectroscopy.Comment: 10 pages, 3 figure
The Near-Infrared Transmission Spectra of TRAPPIST-1 Planets b, c, d, e, f, and g and Stellar Contamination in Multi-Epoch Transit Spectra
The seven approximately Earth-sized transiting planets in the
\object{TRAPPIST-1} system provide a unique opportunity to explore habitable
zone and non-habitable zone small planets within the same system. Its habitable
zone exoplanets -- due to their favorable transit depths -- are also worlds for
which atmospheric transmission spectroscopy is within reach with the Hubble
Space Telescope (HST) and with the James Webb Space Telescope (JWST). We
present here an independent reduction and analysis of two \textit{HST} Wide
Field Camera 3 (WFC3) near-infrared transit spectroscopy datasets for six
planets (b through g). Utilizing our physically-motivated detector charge trap
correction and a custom cosmic ray correction routine, we confirm the general
shape of the transmission spectra presented by \textbf{\citet{deWit2016,
deWit2018}}. Our data reduction approach leads to a 25\% increase in the usable
data and reduces the risk of confusing astrophysical brightness variations
(e.g., flares) with instrumental systematics. No prominent absorption features
are detected in any individual planet's transmission spectra; by contrast, the
combined spectrum of the planets shows a suggestive decrease around
1.4\,\micron similar to an inverted water absorption feature. Including
transit depths from \textit{K2}, the SPECULOOS-South Observatory, and
\textit{Spitzer}, we find that the complete transmission spectrum is fully
consistent with stellar contamination owing to the transit light source effect.
These spectra demonstrate how stellar contamination can overwhelm planetary
absorption features in low-resolution exoplanet transit spectra obtained by
\textit{HST} and \textit{JWST} and also highlight the challenges in combining
multi epoch observations for planets around rapidly rotating spotted stars.Comment: 41 pages, 20 figures. Accepted for publication on AJ on 8/28/2018.
Comments and suggestions are welcom
Can 1D radiative equilibrium models of faculae be used for calculating contamination of transmission spectra?
The reliable characterization of planetary atmospheres with transmission
spectroscopy requires realistic modeling of stellar magnetic features, since
features that are attributable to an exoplanet atmosphere could instead stem
from the host star's magnetic activity. Current retrieval algorithms for
analysing transmission spectra rely on intensity contrasts of magnetic features
from 1D radiative-convective models. However, magnetic features, especially
faculae, are not fully captured by such simplified models. Here we investigate
how well such 1D models can reproduce 3D facular contrasts, taking a G2V star
as an example. We employ the well established radiative magnetohydrodynamic
code MURaM to obtain three-dimensional simulations of the magneto-convection
and photosphere harboring a local small-scale-dynamo. Simulations without
additional vertical magnetic fields are taken to describe the quiet solar
regions, while simulations with initially 100 G, 200 G and 300 G vertical
magnetic fields are used to represent different magnetic activity levels.
Subsequently, the spectra emergent from the MURaM cubes are calculated with the
MPS-ATLAS radiative transfer code. We find that the wavelength dependence of
facular contrast from 1D radiative-convective models cannot reproduce facular
contrasts obtained from 3D modeling. This has far reaching consequences for
exoplanet characterization using transmission spectroscopy, where accurate
knowledge of the host star is essential for unbiased inferences of the
planetary atmospheric properties.Comment: 7 pages, 2 figures, submitted to APJ
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